On 7/28/22 10:25 AM, Hao Luo wrote:

On Wed, Jul 27, 2022 at 10:49 PM Yonghong Song [off-list ref] wrote:

quoted

On 7/22/22 10:48 AM, Yosry Ahmed wrote:

quoted

From: Hao Luo <redacted>

Cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:

   - walking a cgroup's descendants in pre-order.
   - walking a cgroup's descendants in post-order.
   - walking a cgroup's ancestors.

When attaching cgroup_iter, one can set a cgroup to the iter_link
created from attaching. This cgroup is passed as a file descriptor and
serves as the starting point of the walk. If no cgroup is specified,
the starting point will be the root cgroup.

For walking descendants, one can specify the order: either pre-order or
post-order. For walking ancestors, the walk starts at the specified
cgroup and ends at the root.

One can also terminate the walk early by returning 1 from the iter
program.

Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
program is called with cgroup_mutex held.

Currently only one session is supported, which means, depending on the
volume of data bpf program intends to send to user space, the number
of cgroups that can be walked is limited. For example, given the current
buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
cgroup, the total number of cgroups that can be walked is 512. This is

PAGE_SIZE needs to be 4KB in order to conclude that the total number of
walked cgroups is 512.

Sure. Will change that.

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quoted

a limitation of cgroup_iter. If the output data is larger than the
buffer size, the second read() will signal EOPNOTSUPP. In order to work
around, the user may have to update their program to reduce the volume
of data sent to output. For example, skip some uninteresting cgroups.
In future, we may extend bpf_iter flags to allow customizing buffer
size.

Signed-off-by: Hao Luo <redacted>
Signed-off-by: Yosry Ahmed <redacted>
Acked-by: Yonghong Song <redacted>
---
   include/linux/bpf.h                           |   8 +
   include/uapi/linux/bpf.h                      |  30 +++
   kernel/bpf/Makefile                           |   3 +
   kernel/bpf/cgroup_iter.c                      | 252 ++++++++++++++++++
   tools/include/uapi/linux/bpf.h                |  30 +++
   .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
   6 files changed, 325 insertions(+), 2 deletions(-)
   create mode 100644 kernel/bpf/cgroup_iter.c

This patch cannot apply to bpf-next cleanly, so please rebase
and post again.

Sorry about that. Will do.

quoted

quoted

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a97751d845c9..9061618fe929 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -47,6 +47,7 @@ struct kobject;
   struct mem_cgroup;
   struct module;
   struct bpf_func_state;
+struct cgroup;

   extern struct idr btf_idr;
   extern spinlock_t btf_idr_lock;

@@ -1717,7 +1718,14 @@ int bpf_obj_get_user(const char __user *pathname, int flags);
       int __init bpf_iter_ ## target(args) { return 0; }

   struct bpf_iter_aux_info {
+     /* for map_elem iter */
       struct bpf_map *map;
+
+     /* for cgroup iter */
+     struct {
+             struct cgroup *start; /* starting cgroup */
+             int order;
+     } cgroup;
   };

   typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index ffcbf79a556b..fe50c2489350 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -87,10 +87,30 @@ struct bpf_cgroup_storage_key {
       __u32   attach_type;            /* program attach type (enum bpf_attach_type) */
   };

+enum bpf_iter_cgroup_traversal_order {
+     BPF_ITER_CGROUP_PRE = 0,        /* pre-order traversal */
+     BPF_ITER_CGROUP_POST,           /* post-order traversal */
+     BPF_ITER_CGROUP_PARENT_UP,      /* traversal of ancestors up to the root */
+};
+
   union bpf_iter_link_info {
       struct {
               __u32   map_fd;
       } map;
+
+     /* cgroup_iter walks either the live descendants of a cgroup subtree, or the
+      * ancestors of a given cgroup.
+      */
+     struct {
+             /* Cgroup file descriptor. This is root of the subtree if walking
+              * descendants; it's the starting cgroup if walking the ancestors.
+              * If it is left 0, the traversal starts from the default cgroup v2
+              * root. For walking v1 hierarchy, one should always explicitly
+              * specify the cgroup_fd.
+              */

I did see how the above cgroup v1/v2 scenarios are enforced.

Do you mean _not_ see? Yosry and I experimented a bit. We found even

Ya, I mean 'not see'...

on systems where v2 is not enabled, cgroup v2 root always exists and
can be attached to, and can be iterated on (only trivially). We didn't
find a way to tell v1 and v2 apart and deemed a comment to instruct v1
users is fine?

So, cgroup_fd = 0, start from cgroup v2 root.
     cgroup_fd != 0, start from that particular cgroup (cgroup_v1 or v2)
Okay, since cgroup v2 root is always available and can be iterated,
I think comments should be okay.

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+             __u32   cgroup_fd;
+             __u32   traversal_order;
+     } cgroup;
   };

   /* BPF syscall commands, see bpf(2) man-page for more details. */

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